The invention relates to the field of video signal processing, and in particular to interpolation of an image information value for a pixel of an interline situated between two original image lines in an image.
For the transmission of video images or television images, it is known how to transmit interlaced fields instead of frames in which an image information value is present for each pixel of the image. Each of these frames transmitted contains a number of scanning lines, each of which contain a number of pixels. Image information values, i.e., luminance values or chrominance values, are only present for the pixels of every other line, and frames are transmitted alternatingly in which image information values are present for pixels of even numbered lines and for pixels of odd numbered lines.
In order to turn such a transmitted field into a frame for representation on a display device, such as a monitor screen, it is necessary to interpolate image information values for the interlines. In so-called intra-field algorithms, the interpolation of an image information value for a pixel of an interline of a field only uses image information values of pixels from the same field.
In the most elementary case, the available image lines of a field are doubled to produce a frame. The image information values of the pixels of an interline interpolated in this way then correspond to the image information values of the image lines lying above or below the interline. Edges running in the diagonal direction through an image being depicted will, however, appear as “steps” in the frame with this kind of interpolation. One method for avoiding such steplike artifacts in a frame generated from a field is described, for example, in U.S. Pat. No. 5,625,421.
In the Edge Based Line Average (ELA) algorithm, for each pixel to be interpolated one determines the direction of a possible edge containing this pixel. The algorithm is described, for example, in T. Doyle, “Interlaced to Sequential Conversion for EDTV Applications” Proc. of 2nd Int. Workshop on Signal Processing of HDTV, L'Aquila, Italy, 1988, or in T. Doyle, and M Looymans, “Progressive Scan Conversion Using Edge Information” Proc. of 3rd Int. Workshop on HDTV, Torino, Italy, 1989, or in Lee et al.: “A New Algorithm for Interlaced to Progressive Scan Conversion Based on Directional Correlations and its IC design,” IEEE Transactions on Consumer Electronics, Volume 40, Number 2, May 1994, page 119.
In the ELA algorithm, one determines the difference, for various image directions, between the image information values of two pixels lying adjacent to the pixel being interpolated in the particular direction. The direction for which the magnitude of this difference is a minimum is used as the direction of the edge contour. The image information value of the pixel being interpolated is then interpolated by making use of pixels situated adjacent in this direction to the pixel being interpolated. Ambiguities can present a problem in this method, as will be explained hereinbelow with reference to FIGS. 1 and 2.
FIG. 1 shows a segment with two original image lines y−1, y+1 of a matrix-type field. Each of these lines has a number of pixels to which image information values, such as luminance values, are assigned. In FIG. 1, y denotes an interline being interpolated. Let us now consider a pixel 15 being interpolated at an image position (x,y). The arrows denoted 1 to 5 illustrate various image directions in FIG. 1 for which a possible edge contour is being investigated. The presence of an edge in a direction is assumed if pixels lying adjacent to the pixel 15 being interpolated in the particular direction have the same or approximately the same luminance values. As can be seen from FIG. 1, which represents the different luminance values of the individual pixels by different shading patterns, various directions for which this criterion is fulfilled may exist. In FIG. 1, the aforementioned “edge criterion” is fulfilled for all five directions shown, whereas in fact the edge runs in the direction illustrated by arrow 4.
In FIG. 2, direction values for different image directions are plotted generally for a pixel being interpolated. The individual direction values correspond here to the magnitude of the difference of the image information values of those original pixels lying adjacent to the pixel being interpolated in the respective direction dx. The presence of an edge in an image direction is all the more probable as the corresponding direction value is smaller. As already explained with reference to FIG. 1, the trend of the direction values q(x,y) can still have ambiguities in the sense that the direction values for two or more image directions dx1, dx2 form local minima of the curve, so that two or more image directions dx1, dx2 can be considered as possible directions for edge contours and, thus, possible directions of interpolation.
Other methods of interpolation of an image information value are described in U.S. Pat. No. 6,965,705, EP 0 550 231, published U.S. Patent Applications 2005/0157951 and US 2005/0134602, PCT application WO 99/19834, or in FAN, YU-CHENG; et al.: “Intelligent Intra-Field Interpolation for Motion Compensated Deinterlacing”, 3rd International Conference on Information Technology ITRE 2005. 27-30 Jun. 2005, pp. 200-203.
There is a need for an interline interpolation that ensures an improved interpolation technique of edges running through the image.